One of the by-products created by the combustion of hydrocarbon (HC) fuels in burners that use atmospheric air is nitrogen oxides (NOx). NOx emissions have become a highly regulated pollutant in many industrial processes including steel and aluminum manufacture. Efforts to reduce fuel consumption through combustion air preheating have led to exponential increases in NOx levels from conventional burner designs, which has been well known in the industry for many years. Efforts to save fuel and increase combustion efficiency via recuperative and/or regenerative combustion systems combined with stricter governmental permitting laws for acceptable NOx emissions from furnaces has led to a much greater awareness and need to solve this problem in recent years.
Techniques for controlling and inhibiting NOx formation in furnace combustion processes may include provisions for staging fuel, staging combustion air, recirculating flue gas into the burner, recirculating flue gas into the burner flame, altering combustion patterns with different degrees of swirl, and injection of water or steam into the burner or flame. Factors that contribute to the formation of NOx in burner fired combustion chambers are the oxygen content of the flame or combustion chamber, the temperature of the combustion chamber, the air preheat temperature, and the burner firing rate.
It is known that NOx emissions increase with combustion chamber temperatures, the temperature of the combustion air, and with oxygen content in the combustion chamber. However, these factors are difficult to predict because burners for different industrial processes operate at various furnace chamber temperatures, have various oxygen concentrations in the work chambers, may or may not have preheated combustion air, and are required to operate at different heat inputs depending of changing heat load requirements.
Previous efforts to solve the problem include the Staged Air, Low NOx Burner with Internal Recuperative Flue Gas Recirculation, U.S. Pat. No. 5,413,477. This design utilizes a combination of air staging and flue gas recirculation (FGR) for NOx reduction. However, the added capital expense for piping and controlling the recirculated flue gases are substantial in this design.
Another burner design by Bloom Engineering Co., Inc., International Patent No. WO 01/35022 A1, includes an air staged burner for lower NOx emissions, but does not address the cold furnace startup issue. In addition, there is still a need to reduce burner NOx emissions beyond that achieved by this burner.
Finally, Tokyo Gas Co., Ltd., U.S. Pat. No. 5,571,006, describes an air staging method for lower NOx emissions from burners including regenerative beds in the burner body. However, this design requires a separate ambient air connection to the burner body for flame stabilization and complete fuel burnout during cold furnace startups at/or below the auto ignition temperature of the fuel requiring added maintenance, installation, and operation costs.
In addition to not reducing the NOx emissions to an acceptable reduced value, these prior art burner designs also fail to produce a flat stable flame from low fire to high fire values. Low fire values are typically about ⅛ of high fire values. The production of a flat stable flame is important to provide a uniform heat flux to the material being heated.
A previous effort to reduce NOx emissions is disclosed in the Three Stage Low NOx Burner and Method, U.S. Pat. No. 7,4163,392, incorporated herein in the entirety by reference. This design utilizes a combination of air staging to provide for the first stage of combustion taking place substantially within a burner cup section of a refractory baffle, and the second stage of combustion taking place adjacent to the baffle and downstream of the baffle, and the third stage of combustion taking place substantially within the furnace. However, there is a continuing need to further reduce NOx emissions and improve flame stability with flat flame characteristics.
What is needed is a gas burner that is capable of very low NOx emissions when fired on either ambient or preheated combustion air. The burner should also reduce carbon monoxide CO and hydrocarbon HC emissions during cold furnace startups and reduce emissions without the added expense of multiple air and/or fuel connections. Additionally, the burner should provide a flat stable flame capable of providing a uniform heat flux to the furnace at both low temperature and high temperature operating conditions over a wide turndown range.